Square root function working

Evaluation of unary operators now occurs right to left, which makes it
possible to have things like square roots. However, I still can’t do
factorials, because parsing from right to left only allows for left
unary operators. Still haven’t fixed the lexing issue with ‘sixty six’.
Sixty six finally is lexed properly!

Achieved this by sorting operators.keySet() in descending order before
going through it to look for matches.